1. Field of the Invention
The present invention relates to a method and apparatus for calibrating and correcting magnetic and geometrical distortions in an imaging system, and, more specifically, to calibrating and correcting magnetic and geometrical distortions in an image intensifier-based computed tomography (CT) system.
2. Description of the Related Art
An image intensifier-based CT system utilizes an image intensifier to detect x-rays emitted by an x-ray source after they are passed through an object under test. The image intensifier converts x-ray photons to optical photons to electrons, which are accelerated and focused down the length of the image intensifier onto a small phosphor screen. The phosphor screen generates an image which is recorded by an optical sensor which outputs a video signal typically formatted into 525 lines of video information. This video signal is then digitized for further processing.
A two-dimensional slice image of an object under test is reconstructed from a plurality of the image frames corresponding to a plurality of views of the object under test taken as either the object under test is rotated 360.degree. on a turntable (turntable rotation) or as the x-ray source and image intensifier are rotated 360.degree. around the stationary object under test (gantry rotation). For example, if the system produces 30 image frames per second and the turntable or gantry completes one rotation every 60 seconds, the slice image will be reconstructed from 1800 separate image frames corresponding to the 1800 distinct views of the object under test.
Image intensifiers impart two distortions to the positions of detected x-ray photons. The first distortion is due to the construction geometry of the image intensifier and is referred to as "geometrical" distortion. This distortion causes a pagoda-like distortion of the detected image frame. That is, if a set of nested squares are formed by wires and placed on the entrance surface of the image intensifier, the output image appears as shown in FIG. 1. This is also referred to as "pin-cushion" distortion.
The second distortion is caused by the magnetic force of the earth, and will be found in any imaging system using an x-ray detector having an electron tube. Because the x-ray detector (image intensifier) accelerates electrons down its length, these electrons are subject to magnetic distorting forces of the form: EQU F=q v x B, (1)
where F is the distorting force vector;
q is the elementary charge of the electron;
v is the velocity vector of the electron; and
B is the magnetic field vector of the earth.
This distorting force further twists the image of FIG. 1.
If the imaging system is a turntable rotation system, these two distortions are constant for each image frame generated by the image intensifier. However, if the imaging system is a gantry rotation system, the magnetic distortion differs for each image frame generated by the image intensifier. The magnetic distorting force distorts each image frame differently due to the different orientation of the image intensifier as the gantry rotates.
Additional magnetic distortions result in any imaging system using a tube TV camera as an optical sensor as a result of the moving electrons in the TV imaging tube.